1. Field of the Invention
The present invention relates generally to semiconductor devices sealed with molding resin, and, more particularly, to a semiconductor device sealed with molding resin which is improved to have enhanced reliability. The present invention further relates to a method of manufacturing such a semiconductor device sealed with molding resin.
2. Description of the Background Art
As miniaturization of semiconductor elements progresses, volatilization failure or malfunctions of elements caused by the stress of sealing resin becomes a problem in conventional semiconductor devices sealed with resin. In order to solve this problem, a method in which a stress buffering film of a resin layer is formed on an element is proposed.
Conventionally, polyimide is often used for the resin layer. A conventional example using polyimide as a stress buffering film will be described.
FIG. 1 is a cross sectional view of a semiconductor device disclosed in Japanese Patent Laying-Open No. 56 -118334 (1981). Referring to FIG. 1, an aluminum interconnection pattern 3 is formed on a semiconductor chip 1. Aluminum interconnection pattern 3 includes an aluminum electrode pad 3a. A phospho-silicate glass (PSG) film 8 is formed on semiconductor chip 1 to cover aluminum interconnection pattern 3. An opening is provided in PSG film 8 for exposing the surface of aluminum electrode pad 3a. A polyimide film 9, which is to serve as a stress buffering protective film for buffering a stress applied to an element, is provided on semiconductor chip 1. Although not shown in the drawing, a gold wire is bonded to aluminum electrode pad 3a, and the whole of the semiconductor device is sealed within, for example, an insulating housing.
FIGS. 2A-2D are cross sectional views of a semiconductor device including a polyimide stress buffering film disclosed in Japanese Patent Laying-Open No. 63-104450 (1988) in respective steps of a manufacturing process.
Referring to FIG. 2A, a SiO.sub.2 film 12 is formed on a silicon substrate 11. Although not shown, an element is provided in silicon substrate 11. An aluminum interconnection pattern 3 is formed on SiO.sub.2 film 12. Aluminum interconnection pattern 3 includes an aluminum electrode pad 3a. A SiN film 4 is formed to cover aluminum interconnection pattern 3. An opening 4a is provided in SiN film 4 for exposing aluminum electrode pad 3a in aluminum interconnection pattern 3.
Referring to FIG. 2B, a PSG film 8 is formed over semiconductor substrate 11 by a CVD process to fill opening 4a in SiN film 4.
Referring to FIG. 2C, a polyimide stress buffering protective film 9 is formed on PSG film 8. Polyimide film 9 is selectively etched to form an opening 9a in a part positioned on aluminum electrode pad 3a.
Referring to FIG. 2D, after etching of polyimide film 9, PSG film 8 is etched using a mixed liquid of HF and NH.sub.4 F to expose aluminum electrode pad 3a. Then, although not shown, an Au wire is connected to aluminum electrode pad 3a, and the whole device is sealed with molding resin.
Next, the reason for forming PSG film 8 will be described. If there is no PSG film 8 for covering aluminum electrode pad 3a, an alkaline solution used as an etchant corrodes the surface of aluminum interconnection film 3 during etching of polyimide film 9, and therefore, aluminum electrode pad 3a is made rough. The roughness of aluminum electrode pad 3a causes failure in the connection of aluminum electrode pad 3a to the Au wire when the Au wire is bonded, and therefore, it causes reliability of the semiconductor device to be lowered. PSG film 8 serves for preventing the roughness of the aluminum electrode pad.
As described above, a polyimide film is used as a stress buffering protective film in the prior art.
However, in the case of the prior art illustrated in FIG. 1, a tensile stress is generated in polyimide film 9 during formation of polyimide film 9 (by a curing reaction). After formation of the film, polyimide film 9 has a residual stress of about 5.times.10.sup.8 dyn/cm.sup.2. Referring to FIG. 1, this tensile stress causes stress migration of aluminum in underlying aluminum interconnection pattern 3. Therefore, defects in the shape of a wedge are generated in aluminum interconnection 3 during formation of polyimide film 9, and there is a problem of decline in reliability of the semiconductor device.
In addition, according to the conventional example illustrated in FIGS. 2A-2D, it is necessary to form PSG film 8 covering aluminum electrode pad 3a for preventing aluminum electrode pad 3a from being rough during etching of polyimide film 9, and it is also necessary to provide the opening in PSG film 8 for exposing aluminum electrode pad 3a, so that there is a problem of complexity of the process as a whole.